They are where the expertise that lives in individual engineers gets preserved in forms that survive those engineers.

The Planetary Flight Handbook was that kind of document.

The calculations it contained were not created for the handbook — they were developed through years of work on specific aerospace problems, and the handbook organized and codified that work in a form that mission planners could use.

It was, in the language of engineering, a reference document derived from research.

The people who co-authored it were the people who had done the research.

Who had worked through the orbital mechanics, the trajectory options, the performance constraints, the mission design trades.